He served two years in the United States Department of Justice in the Honor Law Graduate Program from 1961 to 1963. He entered private practice in Evanston, Illinois from 1963 to 1979. During this time he entered politics; first by becoming a member of the Illinois House of Representatives from 1973 to 1979,[1] then by running an unsuccessful first campaign for a seat in the Ninety-sixth United States Congress in 1978 against Representative Abner J. Mikva. When Mikva resigned from Congress in 1979, Porter won a special election to succeed him and continued to serve for the next ten Congresses. He was a Representative in total from January 22, 1980 to until his retirement on January 3, 2001.

Currently he is a partner of the law firm Hogan Lovells[1] and serves as chairman of Research ! America, Vice Chair of the Foundation for the National Institutes of Health. He is a member of the Boards of the PBS Foundation and the First Focus Campaign for Children [2], and a trustee for the Brookings Institution. Porter is a member of the Institute of Medicine of the National Academy of Sciences, the Bretton Woods Committee, the InterAmerican Dialogue and Council for Foreign Relations. Previously, he was Chairman of PBS and served on the boards of the RAND Corporation, the American Heart Association and the John F. Kennedy Center for the Performing Arts.

In 2000, he was awarded The Mary Woodard Lasker Public Service Award "for wise and perceptive leadership on behalf of medical research funding and a deep commitment to strengthening the science enterprise." He has also received the Albert Sabin Hero of Science Award from Americans for Medical Progress for his consistent advocacy for medical research.